Screening and comminuting machines, hereafter sometimes referred to as comminutors, are required in waste treatment installations to prevent clogging of valves, pumps, and other apparatus by reducing the size of foreign material that enter the treatment installation together with the liquid wastes which carry the foreign material. Comminution promotes the speed with which the treatment and purification of waste materials is carried out. Efficient and reliable cutting of suspended solids is essential and, at the same time, the comminutor must be simple in construction, economical in cost of operation, and require a minimum maintenance and servicing effort since the machines may be installed in locations which are not readily accessible.